Process for preparing precipitated calcium carbonates

ABSTRACT

A process for the preparation of precipitated calcium carbonate by introducing CO 2  into milk of lime, in which the milk of lime is first ground, and then the CO 2  is introduced to precipitate the calcium carbonate.

BACKGROUND OF THE INVENTION

[0001] The invention relates to a process for the preparation ofprecipitated calcium carbonate which is suitable as a functional fillerand is distinguished by defined properties. One typical field ofapplication is the use of the precipitated calcium carbonate as a fillerin plastisols, such as underbody sealants for automobiles.

[0002] The precipitated calcium carbonate in this case is capable ofcontrolling the flow behavior of the plastisols. Thus, by using thecalcium carbonates which are suitable in each case, plastisols havingvery differing flow behaviors can be prepared. Depending on the intendedapplication, high, medium or low flow limits may be desired. The flowlimit according to Bingham in the plasticiser mixture is usually quotedas a typical characteristic for the application-related suitability ofcalcium carbonates as fillers. The ability of the precipitated calciumcarbonate to control the flow limits is influenced, inter alia, by thenature of the burnt lime used for producing the milk of lime and by theslaking operation.

[0003] Finely dispersed calcium carbonate is either prepared fromnaturally-occurring raw materials by mechanical preparation, orchemically, e.g. by precipitation.

[0004] Usually, precipitated calcium carbonate is prepared byintroducing CO₂ into milk of lime

Ca(OH)₂+CO₂→CaCO₃+H₂O

[0005] The properties of the precipitated calcium carbonate depend, forexample, on the chemical composition of the limestone used or on theburning conditions.

[0006] Preferably so-called soft-burned lime is used to prepareprecipitated calcium carbonate. To this end, limestone is burned at atemperature of 1000 to 1250° C.

[0007] Among other characteristics, the ability of burnt lime to reactwith water is used to characterize it. The so-called VEC value (Vélocitéd'Extinction de Chaux) is measured. In this case, the heating rate of adefined water/burnt lime mixture is measured. Typical VEC values forburnt lime which is suitable for the preparation of precipitated calciumcarbonate are above 3° C./min.

[0008] To prepare the milk of lime, the burnt lime is reacted withwater, for example in a slaking drum, whereupon a Ca(OH)₂ suspension,so-called milk of lime, forms. The milk of lime usually has a solidscontent of 70 to 220 g/liter Ca(OH)₂, a viscosity of 30 to 1500 mPas andan average particle diameter of less than 50 μm.

[0009] Milk of lime is used, for example, to neutralize waste water, orto prepare soda or precipitated calcium carbonate.

[0010] One drawback of the previously known processes for preparingprecipitated calcium carbonate is that it has hitherto not been possibleto satisfactorily control the application-related properties of theprecipitated calcium carbonate, in particular as a rheological additive,due to its average grain size, grain size distribution and particleform.

[0011] U.S. Pat. No. 3,920,800 describes a process for the preparationof precipitated calcium carbonate of high purity. According to themethod described, an aqueous calcium hydroxide slurry (milk of lime) isground in the presence of CO₂ at a temperature of at most 100° C. Thecalcium carbonate formed by the reaction of the calcium hydroxide withthe CO₂ is free of calcium hydroxide. The particle size of the calciumcarbonate is controlled by means of the temperature during the grindingprocess. At temperatures of approximately 30° C., very small particlesare obtained, and at temperatures of approximately 80° C. larger calciumcarbonate particles are obtained.

SUMMARY OF THE INVENTION

[0012] It is the object of the invention to provide a process forpreparing precipitated calcium carbonate (CCP) having defined,reproducible properties.

[0013] This and other objects are achieved in accordance with theinvention in that the milk of lime is ground before the CO₂ isintroduced to precipitate the calcium carbonate.

[0014] Known mills may be used for grinding the milk of lime. Preferablybead mills are used. Suitable grinding beads include, in particular,zirconium oxide grinding beads having a diameter of 0.4 to 2 mm.

[0015] The grinding result is greatly influenced by the fineness of thebeads.

[0016] The grinding effect is furthermore controlled by the speed of themill, the residence time of the milk of lime in the mill and theresulting specific energy input.

[0017] The average particle size of the Ca(OH)₂ particles in the milk oflime or the viscosity of the milk of lime can also be measured as ameasured parameter indicating the success of grinding.

[0018] The course of the wet grinding can also be followed by measuringthe grain distribution of the calcium hydroxide or by measuring theviscosity of the milk of lime at a defined concentration andtemperature.

[0019] According to the invention, the viscosity of the milk of lime ismeasured and used to control the mill. The viscosity should be measuredwithout difficulty, e.g. continuously, and the mill throughput or thespeed of the mill, and hence the resulting quality of the milk of lime,can be controlled automatically by means of a corresponding controlcircuit.

[0020] In one embodiment of the invention, the milk of lime is ground toa viscosity of at most 2500 mPas. A higher viscosity should be avoided,since otherwise the introduction of the precipitating gas is notoptimally possible.

[0021] Therefore, for the purposes of the invention, the viscosity ofthe ground milk of lime should be set such that the milk of lime canflow readily, so that the precipitating gas can be dispersedhomogeneously. The grinding furthermore makes it possible to adjust theviscosity of the milk of lime such that calcium carbonate withapplication-related properties which are desired in each case, inparticular as a rheological additive, can be prepared. Thus, dependingon the intended purpose, CaCO₃ can be prepared in a consistent qualitywhich is suitable as a high functionality constituent of mixtures fore.g. polymers, polymer preparations, plastics, coating compounds,sealing compounds, paper or paints, in particular printing inks.

[0022] After grinding, the milk of lime is supplied to a container intowhich CO₂ is introduced to precipitate the calcium carbonate and ishomogeneously dispersed in the milk of lime.

[0023] Surprisingly, it has been discovered that the properties of theprecipitated calcium carbonate can be reproduced if the milk of lime isfirst ground, and then CO₂ is introduced to precipitate the calciumcarbonate.

[0024] If precipitated calcium carbonate from non-ground milk of lime iscompared with that from ground milk of lime, it is apparent that thegrinding has an influence on the reaction rate, on the average particlediameter of the CCP and on the use-related properties of the CCP.

[0025] It has been discovered that a decreasing particle size in themilk of lime also involves a reduction in the average particle diameterof the CCP.

[0026] It has furthermore been discovered that the precipitation time isdecreased by about 20% by using the milk of lime ground according to theinvention.

[0027] The viscosity-controlled wet grinding of the milk of limeaccording to the invention makes it possible to significantly influencethe flow limit of the CCP-filled plastisols, i.e. to adjust themspecifically. It has been found that the flow limit is a function of thespecific energy introduced during grinding, or of the viscosity of themilk of lime after grinding. It was therefore discovered that theproperties of the CCP can be reproducibly influenced by specificallyadjusting the viscosity of the milk of lime.

[0028] The undesirable influencing of the properties of the CCP byundefined fluctuations in the nature of the burnt lime and of theslaking operation are cancelled out by the grinding of the milk of limeaccording to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0029] The following examples are intended to illustrate the inventionin further detail without restricting its scope. Examples 1-6: Burntlime was slaked in a slaking drum: Lump size 2-5 cm VEC: 4.8° C./min Theresulting milk of lime was ground and characterized: solids content 150g/liter average particle diameter D₅₀: 4.4 μm

[0030] Grinding Conditions:

[0031] horizontal attrition mill

[0032] grinding chamber volume 5 liters

[0033] 22 kW motor power

[0034] grinding bodies:

[0035] spheres of zirconium silicate

[0036] diameter 1.0-1.6 mm

[0037] filled volume: 4.8 liters

[0038] The tests were performed at a constant speed of 1990 rpm. Theresidence time of the milk of lime in the mill was varied. Differentspecific energy inputs resulted from this.

[0039] Calcium carbonate was produced from the resulting ground milk oflime by introducing CO₂-containing gas: Precipitation conditions:Precipitation temperature: 18° C. CO₂ concentration in the 30% by volumeprecipitating gas: Gas stream: 1 m³/hour per 10 liters milk of limeDuration of precipitation: see table Subsequent treatment: 2% fatty acidGrinding CCP precipitation CCP Specific energy Milk of lime Duration ofCCP particle (kWh/t) relative Viscosity Milk of lime precipitationdiameter Example to dry product (mPas) D50 (μm) (min) dp (nm) 1  0  454.4 98 70 2  23  50 4.4 94 71 3  31 105 3.2 92 66 4 113 135 2.8 83 62 5254 860 2.1 79 63 6 307 1120  1.8 77 61

[0040] It is apparent that the viscosity of the milk of lime and theaverage particle diameter of the milk of lime can be controlledspecifically by the wet grinding. This means that the duration ofprecipitation of CCP can be reduced to 20%. The particle diameters ofCCP are reproducibly adjusted. Examples 7-12: Preparation of anunderbody sealant: Plasticiser, dioctyl phthalate 55 g Plasticiser,diisononyl phthalate 60 g paste-making PVC, Solvic 374 MIB 70 gpaste-making PVC, Solvic 266 SF 30 g CaCO₃, in accordance with Examples1-6, 70.0 g respectively, coated with fatty acid UV-stabilizer, IRGASTAB17 MOK 2.0 g Adhesion promoter, Euretek 505 4.0 g Drying agent, calciumoxide 5.0 g Example Flow limit (Pa) 7 40 8 48 9 80 10 89 11 157 12 200

[0041] CCP from Examples 1-6, respectively, was used in each case toproduce underbody sealants corresponding to the formulation described.It becomes clear that the flow limit of the CCP-filled plastisol canalso be specifically controlled by wet grinding the milk of lime.Example 13: Preparation of an offset-printing ink Pigment, flushed,Eurolith Blue 25.0% by weight CaCO₃, in accordance with Example 4, 15.0%by weight coated with fatty acid Printing oil, Haltermann, PKWF 4/712.0% by weight Binder, Uroset 48.0% by weight Result Flow limit  31 PaViscosity at shear rate 3/sec 9.4 Pas Depth of color C  53 Gloss 60° 71%

[0042] Example 14: Preparation of polyurethane compounds, 2-componentsystem Polyol, Desmophen 1150 150 g CaCO₃ in accordance with Example 6,60 g coated with fatty acid Titanium dioxide pigment (rutile), 3 g TionaRcl-535 Dry paste, Baylith L Paste 15 g Adhesion promoter, Acronal 700L1 g Plasticiser, Mesamoll II 35 g Polyurethane activator 1 g Isocyanate3 g Result Flow limit according to Bingham 220 Pa Viscosity at shearrate (100/sec) 6.1 Pas Pot life 26 min Adhesion good Dispersionquality/grindometer <35 μm

[0043] Example 15: Preparation of silicone compounds For testing, anon-curing silicone compound was produced which corresponds to a2-component system, but the catalyst required for curing was not added.Silicone polymer 60 g CaCO₃ according to Example 2, 38 g coated withfatty acid Plasticizer 2 g Result Flow limit according to Casson 900 PaViscosity at shear rate 5/sec 280 Pas Dispersion quality/grindometer <35μm

[0044] The foregoing description and examples have been set forth merelyto illustrate the invention and are not intended to be limiting. Sincemodifications of the disclosed embodiments incorporating the spirit andsubstance of the invention may occur to persons skilled in the art, theinvention should be construed broadly to include all variations fallingwithin the scope of the appended claims and equivalents thereof.

What is claimed is:
 1. A process for the preparation of precipitatedcalcium carbonate with reproducible application-related properties, saidmethod comprising the steps of: subjecting milk of lime to specificgrinding treatment to obtain ground milk of lime, and thereafterintroducing CO₂ into the ground milk of lime to precipitate the calciumcarbonate.
 2. A process for the preparation of precipitated calciumcarbonate according to claim 1, further comprising the steps of:continuously measuring the viscosity of the milk of lime during thespecific grinding treatment, and controlling the grinding in response tothe measured viscosity to regulate the quality of the ground milk oflime.
 3. A process for the preparation of precipitated calcium carbonateaccording to claim 1, wherein the specific grinding is controlled toproduce ground milk of lime having a viscosity of at most 2500 mPas. 4.A process for the preparation of precipitated calcium carbonateaccording to claim 2, wherein the specific grinding is controlled toproduce ground milk of lime having a viscosity of at most 2500 mPas.